Arthroplasty is a surgical operation wherein the damaged portions of a joint, such as the knee, are partially or completely replaced with special prostheses made of metal and plastic.
One of the most important aims of knee arthroplasty operations is to restore the original extension of the patient's leg by ensuring that the leg bones are correctly positioned with respect to each other and that the tension of the ligaments on both sides of the knee is balanced. Mistakes in restoring the original anatomic relationships may result in excessive stresses on the knee, which lead to an early failure of the arthroplasty.
The mechanical axis of the leg is an imaginary line on a frontal plane which connects the head of the femur with the center of the ankle and indicates the direction of the load supported by the leg of a person. In a healthy person the mechanical axis of the leg passes through the center of the knee, which ensures that the femoral condyles withstand the same load. In an injured patient the mechanical axis of the leg passes laterally to the center of the knee, thus concentrating a larger part of the load on one of the femoral condyles, which is consequently damaged.
The surface of the knee is enclosed on each side by the collateral ligaments, which in a healthy patient are equally tensioned when the leg is in the extended position. This tension of the lateral ligaments avoids lateral movements of the femur with respect to the tibia, also called varus/valgus movements. In an injured patient the femur and the tibia are not correctly positioned with respect to each other, which causes the mechanical axis of the leg to be bent thus causing a stretch of the collateral ligament located on the convex side of the deformation and a contraction of the collateral ligament located on the concave side.
In order to recover the anatomic functionality of a deformed knee, it is simultaneously required to lengthen the contracted collateral ligament, to tension both collateral ligaments so as to correctly counteract the varus/valgus movements when the leg is extended, and to verify that the mechanical axis of the leg passes through the center of the knee.
To this purpose, the facing ends of the femur and tibia are resected in order to remove their worn out portions and to make room for the components of a prosthesis suitable to correctly restore the anatomic relationships of the joint.
There are known distractor devices for knee arthroplasty that are inserted between the tibia and the femur when the leg is extended in order to move them away from each other, thus temporarily restoring the anatomic relationships of the joint, i.e. the correct position of the mechanical axis of the leg with respect to the knee and the balancing of the tension state of the collateral ligaments. Under these conditions, the surgeon can correctly define the resection levels of the ends of the tibia and femur in order to install the prostheses.
Knee distractor devices for arthroplasty are disclosed e.g. in U.S. Pat. No. 4,501,266 and EP 1915951 and in patent publications WO 98/25526, EP 809969, U.S. 2004/153084 and WO 99/35972.
The resections of the tibia and femur are made by using special slotted jigs which must be fixed on the respective bone by suitable pins. The correct positioning of the jigs in the longitudinal direction of the bone and their orientation with respect thereto are fundamental for a successful arthroplasty and it is therefore necessary to fix suitable reference points.
In the case of the tibia, the resection jig is typically mounted on a telescopic rod that is anchored to the ankle at one end and arranged along the longitudinal direction of the bone by taking the second finger of the foot as a reference. The rod so arranged is aligned with the mechanical axis of the leg. Once the alignment has been carried out, a clamping device is used, which allows the surgeon to correctly position the jig with respect to the end of the tibia in order to resect the desired bone portion. Once the positioning has been carried out, the jig is fixed on the tibia through relevant pins and the telescopic rod and the clamping device are removed in order to make room for the resection tools. The flat surface resulting from the resection is perpendicular to the mechanical axis of the leg.
In the case of the femur the procedure is more complex. In fact, in order to fix the prosthesis it is necessary to carry out multiple resections at the distal end, the first and most important of which must be made transversally to the femur by taking into account the valgism angle of the patient, while the others, generally four, are made on different planes by taking the plane of the first resection as a reference.
The valgism angle is measured during a pre-surgery phase with the help of a radiography of the patient's leg and it represents the angle between the mechanical axis of the leg and the anatomic axis of the femur, the latter passing through its medullary cavity. In a healthy person this angle is comprised between three and eight degrees.
In order to make the distal resections of the femur, it is known to initially bend the leg at 90° in order to drill the femur coaxially to the medullary cavity. A rod suitable to serve as a support for an instrument allowing to mount a first resection jig and to orient it according to the valgism angle measured during the pre-surgery phase is then inserted into the medullary cavity. As in the case of the tibia, the size of the bone portion to be resected is defined with the help of a clamping device connectable to the rod inserted in the medullary cavity of the femur.
Once the first resection has been carried out, the flat surface resulting therefrom is used as a base for placing a second instrument, which allows to fix the mounting pins of a second resection jig provided with four seats. The subsequent resections on the distal end of the femur are then carried out in a direction that is approximately perpendicular to the first one. Also in this phase it is necessary to precisely establish the orientation of the jig with respect to the bone, in particular, considering the plane of the first resection, its rotation angle around the rod inserted in the medullary cavity, in order to avoid mistakes in the resection of the femoral condyles that might lead to a wrong positioning of the prosthesis.
In order to improve the positioning of the surgical instruments used in knee arthroplasty many solutions have been developed.
U.S. 2004/0122441 discloses an apparatus for arthroplasty operations suitable for measuring a joint gap and ligament balance between an osteotomized surface at a femoral distal end and an osteotomized surface at a tibial proximal end. The apparatus is provided with a fixed plate extending from a body and suitable to contact the osteotomized surface at the tibial proximal end. The apparatus further comprises a vertically movable arm connected to the body at the free end of which a supporting plate is rotatably mounted. The supporting plate is so configured to contact the osteotomized surface at the femoral distal end.
EP 327249 discloses a modular apparatus for use in the preparation of bone surfaces and the implantation of a modular total knee prosthesis in a patient. The apparatus comprises resection guides, templates, alignment devices, a distractor and clamping instruments which provide modularity and facilitate bone resection and prosthesis implantation. During the surgical operation, the distractor is fixed to the femur by means of an alignment fork integral with a rod previously fitted into the medullary cavity. Subsequently, alignment devices are mounted on the distractor so fixed in order to determine suitable reference points for the pins supporting the resection jigs.
U.S. 2008/051798 discloses an alternative method for locating the mechanical axis of a long bone, which avoids the use of an intra-medullary rod. The method involves generating an image of the long bone and determining the angle between the mechanical axis of the bone and a reference axis. The method is implemented with a distractor assembly comprising a fixed plate and a mobile plate wherein the mobile plate is translatable by means of an actuator with respect to a body of the distractor assembly between a first position where it is in contact with the fixed plate and a second position where it is spaced therefrom. The mobile plate is also rotatable with respect to the fixed plate and the rotation angle is displayed by means of a graduated scale and an indicator.
In order to achieve a successful arthroplasty, the flat surfaces obtained by means of the resection of the tibia and of the first resection of the femur must result parallel to each other.
Since the rod diameter is lower than the diameter of the medullary cavity of the femur, once inserted into the medullary cavity the rod may be not correctly aligned with the anatomic axis of the femur. Starting from an unreliable reference system it is thus clear that the orientation of the first resection jig may be subject to even large variations with respect to what has been calculated during the pre-surgery phase, thus negatively affecting the restoring of the mechanical axis of the leg and the valgism angle once the prosthesis is installed.
Moreover, neither the known distractors nor the other known surgical tools used in arthroplasty procedures allow to check in situ the correctness of the calculations carried out during the pre-surgery phase on the valgism angle, which contributes to increase the risk of making mistakes when resecting the femur and of malfunction of the installed prosthesis.
EP 327249 in fact discloses a distractor device to be fixed to the femur by means of an intra-medullary rod provided with an engaging means in the form of a fork. The fork lies on a plane defining a specific angle with respect to the rod. Therefore, since the distractor is fixed to the fork, mistakes in the positioning of the rod with respect to the axis of the medullary cavity will lead to a wrong positioning of the distractor, the alignment devices and of resection masks.
On the other hand, U.S. 2008/051798 teaches the use of a graduated scale to allow the surgeon to find the correct position of the mechanical axis of the femur. However, the graduated scale is used in order to bring the femur in a position corresponding to a theoretical position determined pre-operatively. Therefore, mistakes in the pre-surgery calculations will result in a wrong positioning of the femur and consequently in a bad arthroplasty.